An increasing demand for higher density recording capability has arisen for a magnetic recording medium with a magnetic layer comprising a ferromagnetic powder and a binder resin as main components. One of the conventional approaches to obtain higher density recording is to smooth the surface of the magnetic layer.
However, this approach has the drawback in that during running of a magnetic recording medium in which the surface of the magnetic layer has been made smooth, a coefficient of friction between the magnetic layer and a contact part of an equipment is increased and, as a result, the magnetic layer of the magnetic recording medium is occasionally damaged, or the magnetic layer is sometimes peeled apart, from the support even after only short term use.
Particularly in the case of floppy discs, with the recent popularity and wide-spread use of personal computers, floppy discs are used by a wide variety of users. Thus, floppy discs are used under various environments; and, as a result, the magnetic layer of the floppy disc is often subjected to severe high temperature conditions.
When the floppy disc is used under such severe conditions, a ferromagnetic powder sometimes drops particularly from the magnetic layer, causing clogging of a magnetic head.
It is also known that another means to attain high density recording of a magnetic recording medium is to use a ferromagnetic metal powder as the ferromagnetic powder.
However, when a ferromagnetic metal powder is used in a floppy disc, the magnetic layer is more readily damaged by friction with a jacket or jacket liner than is the case in the use of conventional iron oxide, and drop-out is sometimes experienced. This problem becomes marked when the magnetic layer is used under a relatively higher temperature range of 50.degree. to 60.degree. C., or it is used under such conditions that the temperature is cycled between a high temperature level and a low temperature level.
In order to increase running durability of a magnetic layer, it has been conventionally proposed that fatty acid, or an ester of fatty acid and aliphatic alcohol is added to the magnetic layer as a lubricant, thereby decreasing coefficient of friction.
For example, U.S. Pat. No. 3,833,412 discloses addition of monobasic fatty acids having at least 4 carbon atoms, such as capric acid, lauric acid, and oleic acid, as lubricants; U.S. Pat. No. 3,274,111 discloses addition of fatty acid esters such as butyl laurate, lauryl palmitate, and butyl myristate, of monobasic fatty acids having 12 to 16 carbon atoms and aliphatic alcohols having 3 to 12 carbon atoms, as lubricants; and U.S. Pat. No. 4,303,738 discloses addition of tridecyl stearate as a lubricant.
Moreover, addition of higher fatty acid esters of higher alcohols having a branched molecular structure as lubricants is proposed in JP-A-55-157131, and addition of isocetyl stearate as a lubricant is proposed in JP-A-59-186130 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
These lubricants, however, tend to reduce the strength of the magnetic layer, although they are effective in decreasing the coefficient of friction. Thus, the lubricants cannot be considered to be sufficiently satisfactory in protecting the magnetic layer from damage due to jacket liner interfriction.
Moreover, a method of increasing the strength of the magnetic layer by using fiber-based resins having a high modulus of elasticity and a high mechanical strength as binder resins is disclosed in, for example, JP-A-56-13519, JP-A-62-239316, JP-A-58-70424, JP-A-58-70425, JP-A-56-74833, JP-A-60-133527, JP-A-62-34326, JP-A-62-26628, JP-A-59-79428, JP-A-57-135439, JP-A-56-74832 and JP-A-59-188827.
However, the conventional fiber-based binder resins such as nitrocellulose are not sufficiently high in dispersion capability therein for a ferromagnetic powder, and, thus, cannot improve the surface properties of the magnetic layer, and, furthermore, cannot increase the degree of packing the ferromagnetic powder. For these reasons, the use of conventional fiber-based binder resins fail to provide a magnetic recording medium having excellent electromagnetic properties.
Moreover, use of phenoxy resins as binder resins is proposed in, for example, JP-A-1-106325, and JP-A-64-39623, and JP-B-57-36647, and JP-B-57-36648 (the term "JP-B" as used herein means an "examined Japanese patent publication"). However, even if these binder resins are used, the durability of the magnetic layer to contact with the liner of a floppy disc storage jacket cannot be increased sufficiently in a magnetic recording medium using a ferromagnetic metal powder.
Moreover, a method of adding an abrasive (hard grains), such as corundum, silicon carbide, and chromium oxide to the magnetic layer is proposed. This method, however, cannot sufficiently protect the magnetic layer from damages due to contact with the liner of the jacket.